Thermoplastic resin microporous membranes are used widely as a material for separation, selective transmission, isolation of substances, and the like. For example, the usage includes battery separators for lithium ion rechargeable batteries, nickel-metal hydride batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double layer capacitors, various filters such as reverse osmosis filtration membrane, ultrafiltration membrane, microfiltration membrane and the like, moisture permeation waterproof clothes, medical materials and the like. Especially, polyethylene porous membranes are preferably used as separators for lithium ion rechargeable batteries. This is because the membrane exhibits ion permeability due to electrolytic solution impregnation and possesses not only excellent electrical insulating properties, electrolytic solution resistance, and anti-oxidation properties, but also pore blocking effect, which block the electrical current to prevent excessive temperature increase at the temperature range of about 120 to 150° C. in abnormal temperature increase in batteries. However, if the temperature continues to increase even after the pore blocking for some reason, the decrease in viscosity of the polyethylene that configures the membrane and the shrinkage of the membrane may lead to membrane puncture at a certain temperature. This phenomenon is not limited to polyethylene. Even if the other thermoplastic resin is used, this phenomenon cannot be avoided at the temperature equal to or above the melting point of the resin, which configures the porous membrane.
Especially, separators for lithium-ion batteries greatly affect battery characteristics, battery productivity and battery safety, and require good mechanical properties, heat resistance, permeability, dimensional stability, pore blocking characteristics (shut down characteristics), membrane melt-puncture characteristics (melt-down characteristics) and the like. Furthermore, they require improved adhesion to an electrode material for improvement in cycle characteristics of batteries and improved wettability toward electrolytic solution for productivity improvement.
In order to fulfill these requirements, from the viewpoint of shut-down characteristics, methods in which a low-melting point ingredient is added to a polyolefin microporous membrane have been disclosed. However, there have been problems in that the addition of the low melting point resin has a tendency to increase air permeation resistance during manufacturing of the porous membrane, and, moreover, a tendency to decrease the strength of the microporous membrane.
Furthermore, various modified porous layers laminated on a porous layer have been proposed previously from the viewpoint of heat resistance. As modified porous layers, polyamideimide resin, polyimide resin, and polyamide resin, which have both good heat resistance and good wettability toward electrolytic solution, and/or fluorine-based resin, which exhibits good adhesion toward electrodes are preferably used. A modified porous layer described in the present invention refers to a layer that includes resin, which provides or improves at least one of the functions among heat resistance, adhesion to an electrode material, wettability toward electrolytic solution and the like.
Furthermore, it is necessary to increase the area not only of an electrode but also of a separator to fill a container in order to increase battery capacity. Therefore the decrease in thickness of the separator is expected. However, since the thinner porous membrane may experience deformation in its planar directions, a modified porous layer laminated on a thin porous membrane in a battery separator may delaminate during processing, the slit process, or the battery assembly process and the safety may be compromised.
In addition, significant reduction in battery assembly process time is expected to facilitate cost reduction. Therefore an improvement in adhesion, which can withstand such a high-speed process with few problems such as delamination of modified porous layers, is required. However, when the resin included in the modified porous layer is sufficiently permeated into the polyolefin porous membrane, which is a substrate, in order to improve the adhesion, large increases in air permeation resistance have been problematic. Particularly, when a low-melting point ingredient is added to improve shut-down characteristics, there has been a problem in that air permeation resistance further increases, because the pores tend to be closed by heat generated during the modified porous layer formation.
In Patent Document 1, a method of manufacturing a microporous membrane by stretching a layer, which includes low-melting point polymer, and a layer, which does not include low-melting point polymer, is disclosed.
In Patent Document 2, polyvinylidene fluoride is coated on a 9 μm thick polyethylene porous membrane, and a fraction of polyvinylidene fluoride resin penetrates into fine pores of the polyethylene porous membrane to exhibit anchor effect. The resultant composite porous membrane with a peel strength (T-peel strength) of 1.0 to 5.3 N/25 mm at the interface between the polyethylene porous membrane and the polyvinylidene fluoride coating is disclosed.
In Patent Document 3, a heat-resistant porous layer, which contains acrylic resin and a N-vinyl acetamide polymer or thickener of water soluble cellulose derivative and plate-like boehmite, is disposed on a 16 μm thick, corona-discharge treated polyethylene porous membrane. The resultant separator with a 180° peel strength (T-peel strength) of 1.1 to 3.0 N/10 mm at the interface between the polyethylene porous membrane and the heat-resistant porous layer is disclosed.
In Patent Document 4, the method of producing a porous membrane is disclosed, in which polyethylene solution that comprises 30 wt. % of polyethylene composition (20 wt. % of high molecular weight polyethylene (UHMWPE) with a weight average molecular weight of 2.0×106, 80 wt. % of high density polyethylene (HDPE) with a weight average molecular weight of 3.5×105, and antioxidant) and 70 wt. % of liquid paraffin are extruded from an extruder at 190° C., and the extrudate is wound by a chill-roll kept at 50° C. The resultant gel-like product is stretched biaxially to the magnification of 5×5, and the porous membrane is obtained.
In Patent Document 5, the method of producing a microporous membrane is disclosed, in which polyethylene solution similar to that in Patent Document 3 is extruded from an extruder, and the extrudate is wound by a chill-roll kept at 0° C. The resultant gel-like product is stretched biaxially to the magnification of 5×5, and the microporous membrane is obtained.
In Working Example 1 of Patent Document 6, a polyethylene solution that comprises 50 mass % of composition (47.5 mass % of polyethylene with a viscosity-average molecular weight of 200,000, 2.5 mass % of polypropylene with a viscosity-average molecular weight of 400,000 and antioxidant) and 50 mass % of liquid paraffin is extruded from an extruder at 200° C. and the extrudate is wound by a chill-roll kept at 25° C. The resultant gel-like product is stretched biaxially to the magnification of 7×6.4, and a polyolefin resin porous membrane is obtained. A multi-layer porous membrane obtained by laminating a coating layer that comprises polyvinylalcohol and alumina particles on the surface of the polyolefin resin porous membrane is disclosed.
In Working Example 7 of Patent Document 6, a polyethylene solution that comprises 30 wt. % of polyethylene composition (weight average molecular weight of 4,150,000, weight average molecular weight of 560,000, and weight ratio of 1:9, respectively) and 70 wt. % of solvent mix of liquid paraffin and decalin is extruded from an extruder at 148° C. and the extrudate is cooled in a water bath. The resultant gel-like product is stretched biaxially to the magnification of 5.5×11.0, and a polyethylene porous membrane is obtained. A non-water-based separator for a rechargeable battery obtained by laminating a coating layer that comprises meta-type wholly aromatic polyamide and alumina particles on the surface of the polyethylene porous membrane is disclosed.
In Patent Document 8, a polyolefin microporous membrane with an embossed pattern of diagonal lattices obtained by passing a gel sheet before stretching between an embossing roll and a back-up roll is disclosed.
These conventional arts, however, were not sufficiently able to ensure low shut-down temperature and high permeability while providing a modified porous layer, when it is expected to speed up the manufacturing processes and to provide thinner separators, as anticipated by increased safety, lower cost and higher capacity in the near future. Also, these conventional arts, do not sufficiently ensure safety due to partial delamination of the modified porous layers during slit process or battery assembly process. Especially when the polyolefin resin porous membrane, which is a substrate, becomes thinner, it becomes more difficult to ensure safety sufficiently since it is more difficult to achieve enough anchoring of modified porous layers to the polyolefin resin porous membrane.